Automatic implantable cardioverter/defibrillators have been under development since at least the late 1960's. Such a device is described in Re. U.S. Pat. No. 27,757 to Mirowski. A more advanced system is disclosed in U.S. Pat. No. 5,007,422 to Pless et al. which is assigned to the assignee of the present invention and which is incorporated herein by reference. While the technology of cardioverter/defibrillators has advanced significantly, a need still exists for improved lead systems.
As used herein, the term cardioversion generally may be defined as the correction of either ventricular tachycardia or ventricular fibrillation by the discharge of electrical energy into the heart (0.1-40 joules when discharged through internal electrodes). Ventricular tachycardia is an abnormally rapid heart rate (120-180 beats per minute) originating in the heart's main pumping chambers (ventricles) which is regular in periodicity and oftentimes is life threatening to the patient. Ventricular fibrillation is generally a more rapid heartbeat disorder, disorganized and irregular, or non-periodic, and is fatal unless corrected within minutes. This can be accomplished by the discharge of electrical energy through the heart. More specific medical terminology often uses the term cardioversion to mean the synchronized delivery of an electrical shock to the heart to correct ventricular tachycardia. Defibrillation, then, is often referred to as the non synchronized delivery of electrical energy to the heart to correct ventricular fibrillation. Internal cardioversion is usually effective with 0.1 to 3 joules of electrical energy when delivered synchronously with the electrical heartbeat. Internal defibrillation requires 5 to 30 or more joules of electrical energy, depending largely on the electrode system and electrical waveform used.
Many different types of electrode systems have been suggested over the years. Numerous ones of such systems utilize wire mesh epicardial patch electrodes which are typically applied to the heart during open chest surgery. One such system is described in U.S. Pat. No. 4,827,932 to Ideker et al. Major surgery of this type carries with it risks to the patient which should be avoided if possible. The above mentioned Mirowski patent describes an electrode arrangement whereby one electrode is formed on the distal end of an intravascular catheter that is positioned within the right ventricle and a second electrode is positioned on the surface of the chest or sutured under the skin of the chest wall or directly to the ventricular myocardium. Mehra et al. disclose a system in U.S. Pat. No. 4,953,551 wherein a first catheter mounted electrode is located in the apex of the right ventricle (RV) and a second electrode carried on the same catheter is located in the superior vena cava (SVC). A third plate or patch electrode is located subcutaneously outside the chest cavity. The SVC and subcutaneous patch electrodes are electrically interconnected and electric shocks are applied to the heart between this electrode pair and the RV electrode. A similar system is described in U.S. Pat. No. 4,662,377 to Hellman et al. Still other systems provide a separate catheter for the SVC electrode along with the catheter for the RV electrode and the subcutaneous patch electrode. Many such systems include a sensing and pacing electrode on the distal tip of the catheter, distal of the RV electrode. In this configuration, the distal tip electrode and the RV electrode are paired for pacing and sensing functions but are electrically isolated for cardioversion and defibrillation.
While the transvenous lead systems described above appear to be effective, it is desirable to improve the control of the flow of electrical current through the heart during a cardioverting or defibrillating shock to reduce the energy threshold at which such shocks are efficacious. By steering more of the cardioversion or defibrillation current through the ventricular septum such improved efficacy can be achieved.
It is an object of the present invention to provide an improved transvenous lead system electrode configuration.
It is a further object of the invention to provide a cardioversion/defibrillation system with a reduced energy threshold for effective cardioversion and defibrillation shocks.
It is also an object of the invention to provide an automatic implantable cardioverter/defibrillator system with an electrode configuration which will increase the current flowing through myocardial tissue during a cardioversion or defibrillation shock.
It is another object of the invention to provide a method of defibrillating a patient's heart using a lead system having a first electrode in the right ventricle and a second electrode pair in the SVC region and in the left innominate vein.